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Trying any form of fasting can be dangerous if you have diabetes due to the potential to destabilize blood sugar, experts say.

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When the now 46-year-old Mary Roberts from Lockhart, Texas, was diagnosed with type 2 diabetes in 2008, her doctor immediately put her on Metformin (glucophage), a drug to help stabilize blood sugar. “When I got the diagnosis, I guess I wasn’t surprised,” says Roberts, explaining that not only was she overweight but her mom had been on insulin for type 2 diabetes.

Not wanting to be on medication herself for her entire life, Roberts set out on a path to control the diabetes through diet, but a few years of nutrition classes proved unsuccessful in lowering her blood sugar level. It was after her doctor suggested insulin on top of the high dose of Metformin that Roberts switched gears. “I really wanted to find a way to get healthy,” she says.

She found the solution in changing her approach to eating — just not the way she expected. Intermittent fasting (IF) combined with the popular ketogenic diet, which emphasizes dramatically reducing carbohydrate intake, helped her lose weight and lower her A1C. “I feel amazing,” Roberts says.

What Is Intermittent Fasting and How Is It Done?

Although IF has become more popular in recent years, the diet plan isn’t new. In fact, many religions (including Christianity, Judaism, and Islam) have followers who practice fasting of some variety throughout the year. Fasting is often required for blood tests, medical procedures, or surgery. The reason IF has gained so much attention recently is likely due to the release of new diet books plugging the plans and celebrity endorsements. “I think that it has gained popularity because anytime a person drastically cuts calories from their diet, they’re going to lose weight. And we’re so results driven that by seeing that happen we think, This is a great solution,” says Despina Hyde Gandhi, RD, CDE, a dietitian at NYU Langone's Weight Management Program and the president of the Greater New York Dietetic Association.

There are different ways to do IF, including skipping meals and eating only during a certain time period, as well as restricting calories on certain days of the week and eating normally on other days.

Roberts decided to fast by eating two meals a day, but she never follows a strict schedule. “I eat when I’m hungry,” she says.

Within a few months, Roberts' blood glucose level went from over 200 milligrams per deciliter (mg/dL) to around 130 mg/dL, and within eight months, it was within a normal range. Her blood glucose level now is always in the 80s and 90s, and her A1C, a two- to three-month average reading of blood sugar, is 4.8, which is also normal. After 22 months, she lost 106 pounds and has since kept it off. “Sometimes I pinch myself [because] I can’t believe I did this. I turned it around,” she says.

That study was only done in rodents, which doesn’t necessarily mean the same benefits would be seen in humans — nor does it prove that the diet would be safe for people with diabetes. But other research offers clues: One very small observational study, published in April 2017 in the World Journal of Diabetes, suggests short-term daily IF may help improve fasting glucose, weight, and the postmeal blood sugar level in people with type 2 diabetes. Although the study involved just 10 participants, and it was not a randomized controlled trial — the golden standard for research — the findings may hold truth based on what we already know about weight loss. “There’s no denying that anytime you cut calories from your diet, you’re going to see the scale drop,” Gandhi says. That potential aspect of IF can lead to improvements in insulin sensitivity, too, says Michael Mosley, MD, a science journalist and the author of The FastDiet. Further research needs to be conducted to see if IF is truly safe for the larger group of people with type 2 diabetes.

While research on IF for diabetes prevention is also in its early stages, one study published in May 2011 in the International Journal of Obesity found that women who followed an IF diet two days per week and those who followed a more traditional consistent calorie reduction seven days per week lost weight. Those in the two-day group saw the greatest weight loss, yet they had only a modest reduction in fasting insulin, a measure of the insulin level over time, and insulin resistance.

Why Intermittent Fasting May Harm Blood Sugar Control

Nevertheless, experts say that IF carries risks, especially when you have diabetes and need to keep blood sugar stable.

For starters, skipping whole meals can actually result in poorer blood glucose control, not to mention issues like fatigue, low energy during workouts (and thus an increased risk of injury), and medication imbalances. It may also lead to worse diet choices, having the opposite impact on your waistline and blood sugar. People who restrict calories may be more inclined to reach for carb-heavy fare, for instance. “Then their blood sugar is going to go very high and going to be erratic throughout the day,” Gandhi says.

Plus, severely restricting calories or skipping meals can be a hard plan to stick with. “The problem with fasting is it may be more difficult to maintain in the long term,” says Ruth S. Pupo, RDN, CDE, who practices at White Memorial Medical Center in Los Angeles. While losing weight can be beneficial for people with diabetes, as it increases insulin sensitivity, putting weight back on can have the opposite effect, thereby increasing the risk for diabetes complications.

Furthermore, certain people, like pregnant and breast-feeding moms, should avoid IF, as should those with an underlying disease or medical condition, Pupo says. “Anytime you have a higher demand for more nutrition, you don’t want to do a fast,” she says, explaining that both pregnant and breast-feeding moms require extra calories for themselves and their babies, and fasting can cause them to run out of glucose and burn fat, tissue, and muscle. What’s more, if a pregnant mom overproduces ketones (substances that are produced when the body uses fat for fuel), the effect can be harmful to the fetus.

Likewise, for someone with an overactive thyroid, IF can lead to what's called a “thyroid storm,” which causes an increase in temperature, a rapid heart rate, and possibly even cardiac arrest because of the added stress, Pupo says.

Most important, because of the dangers of blood sugar fluctuation that IF may cause, Gandhi personally advises against the diet plan for people with type 2 diabetes. “It’s not great for diabetes because we don’t want to have those dips, those highs and lows in blood sugar. The goal with diabetes is to maintain a steady and stable blood sugar throughout the day,” Gandhi says.

People with diabetes also run the risk of diabetic ketoacidosis, which is a complication of diabetes that occurs when the body can’t produce enough insulin. Insulin brings glucose into the cells, but when the body doesn’t have enough insulin because carbohydrates aren’t accessible during the fasting periods, the body overproduces ketones. “The ketones from burning fat too quickly have built up in their system, and that could damage the kidneys and go to the brain and cause brain swelling,” Pupo says. Diabetic ketoacidosis may even lead to a diabetic coma or death.

For anyone, regardless of whether they have diabetes, cutting out meals and restricting entire food groups can furthermore cause nutritional deficiencies. Without enough nutrition, particularly protein, there’s also a risk for muscle mass loss. “When you really deprive the body of nutrients, your body not only breaks down fat but muscle as well. And our heart is a muscle,” Pupo says.

Steps to Take Before You Try the Popular Diet Plan

Talk to your doctor. Before starting an IF plan, talk to your doctor to come up with a safe approach and management plan for you, as well as to adjust any of your medication doses. Those people on insulin should be particularly careful: If you’re on it and restrict eating, you may be at a greater risk of low blood sugar, which can lead to life-threatening symptoms, such as dizziness, confusion, seizures, loss of consciousness, or even death, according to the American Diabetes Association. “Low blood sugar is very dangerous for a diabetic person,” Gandhi says.

Find a plan that works for you. Plans for IF vary from those that restrict calories two or three days a week and those that restrict eating to certain periods of the day to more strict plans that include fasting for up to 36 hours at a time for 7 or 14 days. The key is to find a plan that you can stick with for the long term.

Be prepared for side effects. It’s common to have headaches, cramps, constipation, or diarrhea, at least initially. “If you’re feeling very unwell, stop. You might be hungry, but you shouldn’t be lethargic or throwing up,” Dr. Fung says.

Eat healthily. Your diet should consist of whole, unprocessed foods, including nonstarchy vegetables, protein, and healthy fats, as well as a multivitamin and plenty of water to prevent dehydration and headaches. A healthy diet will help you lose or manage your weight and keep your blood sugar level steady.

Don’t skip breakfast. Always start the day with a diabetes-friendly breakfast within one to two hours of waking up. “Our blood sugar naturally rises in the morning — that’s how we wake up — and we don’t want that to be followed by a drop. We want to maintain a steady, stable blood sugar throughout the day,” Gandhi says.

Keep your expectations in check. AnIF plan doesn’t work for everybody, and your medical team may not feel that it’s a good fit for you. It’s important to consult these professionals before giving IF a try, as going for long periods without eating when you have diabetes can be dangerous or, at the very least, not produce the results you want. “If you’re recommending something to a patient, you have to look at the whole picture and think, Can this fit into somebody’s life, will they be happy, and will the results outweigh the risks.” Gandhi says. “To me, the answer is no. You will see some weight loss, but it’s not going to be in a healthy way; it’s not going to be in a sustainable way.”

Get support. If you do get the all clear to try IF, experts advise following the plan with a friend or joining an online community or social network to help motivate you to stick with it.

Intermittent Fasting and Diabetes: The Bottom Line

Although IF may help you lose weight, which can help you better control diabetes, it’s important to consult your medical team. Together you can decide what’s most sustainable and safe for you as an individual. Due to the risk of potential blood sugar swings, full-blown IF may not be for you, especially if you aren't controlling the diabetes well. Instead, decreasing your portion sizes, increasing your physical activity between meals, and making healthy food swaps — all of which align with IF — may be a better approach.

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